1. Field of the Invention
This invention relates to synthetic aperture radar systems for use in target recognition and more particularly, the use of optical techniques to record and process SAR data in real time.
2. Description of the Prior Art
In airborne radar imagery for ground mapping, an aircraft carries a side looking antenna that transmits pulses while the aircraft is in level flight. The slant range coordinate, R, is defined in the plane joining the radar trajectory and any target under consideration, in a direction normal to the aircraft flight path. The azimuth or crossrange coordinate is defined in the direction along the flight path. The slant range resolution is a function of the effective transmitted pulse width while the azimuth resolution is set by the dimensions of the antenna, which at best, is diffraction limited. For an antenna with a diameter D, the cross-range resolution is approximately .lambda.R/D where .lambda. is the transmitted RF wavelength. By increasing the antenna diameter, the cross-range resolution can be increased, being limited only by practical antenna size and weight considerations. Airborne platforms, however, impose severe physical limitations on the size of the antenna which can be used and still maintain acceptable flight characteristics of the aircraft. For instance, an antenna might approach the dimensions of the airborne platform as in a conformal array, however, such large sized antennas are obviously incapable of being carried by an aircraft.
Synthetic aperture radar (SAR) is a method of obtaining enhanced cross-range resolution utilizing a small side looking antenna that can be easily carried by an aircraft. The antenna is carried by the aircraft to a sequence of positions in which the antenna, at each position, radiates a pulse and receives and stores the reflected signal. The SAR in effect generates an array in space which can be made many times greater than the airborne platform. The effect is to obtain high resolution imagery from radar data which otherwise would not be obtainable. The stored data is then processed to recreate the image of the illuminated area seen by the radar.
SAR signal processing can be mathematically described as either a correlation or a filtering process which includes all the coherent returns stored during an aperture time. The SAR image reconstruction process thus requires a large data storage and dynamic range capability and the ability to process all these data within one interpulse period. In addition, focusing of the data is necessary for optimum cross-range resolution, and compensation for non-uniform aircraft motion is also desirable. Two general classes of SAR data processing have been used for image reconstruction; optical and digital.
In a conventional optical processing system, as shown in FIG. 1(a), the raw SAR data is recorded on a photosensitive material 12 for storage and subsequent processing to recreate the target image. The radar receiver output is a sequence of reflected range pulses which are used to intensity modulate a cathode ray tube 14 in which the electron beam is swept in synchronism with the returning pulses. The CRT produces a succession of range traces 16 that are projected by lens 18 onto the film 12. The data film 12 moves with a translational speed synchronized to the velocity of the aircraft so that the traces are recorded side-by-side thereby producing a two dimensional format in which the dimension across the film represents range, and the dimension along the film corresponds to the azimuth dimension.
To process the SAR image, the SAR record must first be corrected for depression and divergence angle to obtain mutually collimated virtual images of the cross and slant range data. This is accomplished, as shown in FIG. 1(b), by use of a conical lens 20 curved in the azimuth direction and a cylindrical lens 22 curved in the range direction. The film 12 having the raw SAR data thereon is illuminated by a spatially coherent plane wave. The plane wave is diffracted by the traces which act as one dimensional Fresnel zone-plate lenses. The azimuth focal plane of the diffracted wave, which is tilted due to the angle of the radar signals with respect to the flight path, is erected and moved to infinity by the conical lens 20. The cylindrical lens 22 operates only in the range dimension and images the signal film plane at infinity. The spherical lens 24 operates in both dimensions, takes the image at infinity and reimages it at its focal plane where the image is sharply focused in each dimension. The spherical lens focuses both locus planes from infinity onto the image film 26 which is moving synchronously with the data film 12. The image film is exposed by means of a vertical slit 28 in an opaque screen 30 to remove parallax effects to imaged targets at different azimuth positions. A discussion of the conventional SAR recording and processing techniques may be found in Cutrona, et al. On the Application of Coherent Optical Processing Techniques to Synthetic Aperture Radar, Proceedings of the IEEE, Vol. 54, No. 8, August, 1966.
The optical recording and processing of synthetic aperture radar signals have certain deficiencies. The chemical film development and handling is usually carried out in a ground based vehicle rather than the aircraft. The resulting time lag between the data acquisition and the image formation is unacceptable in many applications.
U.S. Pat. No. 3,787,840 discloses a method and apparatus for recording synthetic aperture radar data for processing the data in a radar optical correlator for target recognition. A finite length of film extending between reels is utilized to record the SAR data utilizing the conventional CRT technique and to subsequently process the data as the film moves through and optical correlator system. While the system provides for the recording and processing of SAR data suitable for an airborne environment, the system is limited by the length of film available. In many applications, such as military target recognition, there is a need for a continuous on line real time recording and processing synthetic aperture radar system.